Heterostructure for electronic power components, optoelectronic or photovoltaic components

ABSTRACT

The present invention relates to a support for the epitaxy of a layer of a material of composition Al x In y Ga (1-x-y) N, where 0≦x≦1, 0≦y≦1 and x+y≦1, having successively from its base to its surface; a support substrate, a bonding layer, a monocrystalline seed layer for the epitaxial growth of the layer of material Al x In y Ga (1-x-y) N. The support substrate is made of a material that presents an electrical resistivity of less than 10 −3  ohm·cm and a thermal conductivity of greater than 100 W·m −1 ·K −1 . The seed layer is in a material of the composition Al x In y Ga (1-x-y) N, where 0≦x≦1, 0≦y≦1 and x+y≦1. The seed and bonding layers provide a specific contact resistance that is less than or equal to 0.1 ohm·cm −2 , and the materials of the support substrate, the bonding layer and the seed layer are refractory at a temperature of greater than 750° C. or even greater than 1000° C. The invention also relates to methods for manufacturing the support.

FIELD OF THE INVENTION

The present invention relates to a support for producing a heterostructure for the manufacture of electronic power components, optoelectronic components, or photovoltaic components having a support substrate, a bonding layer, a crack-free monocrystalline layer, known as the “active layer” of a material with a composition of Al_(x)In_(y)Ga_((1-x-y))N, where 0≦x≦1, 0≦y≦1 and x+y≦1, presents a thickness of between 3 and 100 micrometers, in which or upon which the power components can be manufactured, and the methods of forming such heterostructures with or without components.

BACKGROUND OF THE INVENTION

For the vertical or planar electronic power device (MOS components, bipolar transistors, J-FET, MISFET, Schottky or PIN diodes, thyristors), optoelectronic component (Laser, LED) and photovoltaic component (solar cells) market, it is interesting to utilize an Al_(x)In_(y)Ga_((1-x-y))N (where x is equal to or between 0 and 1, y is equal to or between 0 and 1, where x+y is less than or equal to 1) conducting substrate and preferably a bulk GaN (or “freestanding”) substrate. These substrates are, however, difficult to manufacture with current technologies and remain very expensive.

A proposed alternative consists of a heterostructure comprising a thick active layer of Al_(x)In_(y)Ga_((1-x-y))N (preferably in doped GaN) formed on a conductive substrate. But the growth of thick layers with a good crystalline quality is still difficult with current methods if the seed substrate is not of the same material as the material epitaxied.

The epitaxy of a thick layer of GaN (approximately 10 micrometers) on a seed substrate such as doped Si or SiC, due to the differences in the coefficient of thermal expansion (CTE) and lattice parameter between the materials, leads to the formation of defects and cracks in the layer which reduces the effectiveness of the electronic, optical or optoelectronic devices formed on this material.

In addition, as document WO 01/95380 discloses, this epitaxy necessitates the utilization of a buffer layer—for example a layer of AlN—between the seed substrate and the GaN that presents high electric resistance. The epitaxy of a thick layer of GaN on a sapphire substrate followed by the transfer of the layer to a conductive substrate by laser detachment is an expensive process. In addition, the choice of these materials does not allow a dislocation density of less than 10⁷ cm⁻² to be reached in the active layer.

In addition, the layer thus formed presents very significant bending which necessitates long preparation steps (polishing, etc.) so that it may be bonded and transferred to a final substrate.

In addition, the transfer of a layer of GaN from a bulk substrate by the Smart Cut™ technology does not enable the desired thicknesses to be reached in a satisfactory manner to date.

Document US 2008/0169483 describes the formation of an epitaxy substrate comprising a seed layer of GaN transferred by the Smart Cut™ technology to a support substrate. A layer of conductive GaN is then deposited on the seed layer and then it is transferred to a thermally and electrically conductive support. This method is complex since it involves two transfers of the active layer of GaN to form the final conductive structure.

Therefore one seeks to design a heterostructure for electronic power components, optoelectronic components or photovoltaic components and a method of manufacturing the heterostructure in view of obtaining a thick, crack-free monocrystalline layer of a material of composition Al_(x)In_(y)Ga_((1-x-y))N on a support substrate, not presenting the disadvantages of methods from the prior art.

More precisely, the heterostructure must present the following properties:

a thick active layer, i.e., with a thickness greater than or equal to 3 micrometers, preferably greater than 10 micrometers,

an active layer comprising a main portion with a thickness representing between 70 and 100% of the thickness of the active layer, the main portion being weakly doped so as to enable a dispersion of the electric field over the thickness of the active layer,

a high thermal conductivity of the heterostructure, i.e., typically greater than or equal to 100 W/m·K (also written as W·m⁻¹·K⁻¹), and

a low dislocation density in the active layer, i.e., of less than or equal to 10⁷ cm⁻².

Thus, there is a need for a support adapted for epitaxy of the active layer in view of forming the heterostructure described above. More precisely, this epitaxy support must enable growth of the thick active layer without forming cracks and must in addition present electric properties compatible with the intended applications. Such a support is now provided by the present invention.

SUMMARY OF THE INVENTION

The support of the present invention enables and facilitates the epitaxial growth of an active layer of a material that has a composition of Al_(x)In_(y)Ga_((1-x-y))N, where 0≦x≦1, 0≦y≦1 and x+y≦1. The heterostructure formed by growing such an active layer on the support substrate represents an embodiment of the invention as do the methods for manufacturing the support(s) and heterostructure(s).

The support generally comprises a support substrate and a bonding layer formed on the support substrate; a monocrystalline seed layer on the bonding layer, wherein the seed layer is for the epitaxial growth of an active layer of composition Al_(x)In_(y)Ga_((1-x-y))N, where 0≦x≦1, 0≦y≦1 and x+y≦1. In a preferred embodiment of the invention, the seed layer can also be a material of the composition Al_(x)In_(y)Ga_((1-x-y))N, where 0≦x≦1, 0≦y≦1 and x+y≦1. The support substrate advantageously has an electrical resistivity of less than 10⁻³ ohm·cm and a thermal conductivity of greater than 100 W·m⁻¹·K⁻¹, and provides a specific contact resistance between the seed layer and the bonding layer that is less than or equal to 0.1 ohm·cm⁻². The seed layer can be doped with a concentration of dopants of between 10¹⁷ and 10²⁰ cm⁻³, wherein the dopants can be n type dopants. The bonding layer is typically a material having an electrical resistivity of less than or equal to 10⁻⁴ ohm·cm, and the seed layer on the support substrate has an electrical resistivity of between 10⁻³ and 0.1 ohm·cm.

The materials of the support substrate, the bonding layer and the seed layer can preferably be refractory at a temperature of greater than 750° C., or more preferably the support substrate, bonding layer and seed layer are refractory at a temperature of greater than 1000° C. The material of the support substrate is preferably refractory metal chosen from the group consisting of tungsten, molybdenum, niobium or tantalum and their binary, ternary or quaternary alloys, wherein the preferred metal alloys are TaW, MoW, MoTa, MoNb, WNb or TaNb. In a preferred embodiment, the support substrate is a TaW alloy comprising at least 45% tungsten or a MoTa alloy comprising more than 65% molybdenum.

The material of the bonding layer can comprise polycrystalline silicon, a silicide, a refractory metal, a metal boride, zinc oxide, or indium tin oxide, wherein the silicide is preferably tungsten silicide or molybdenum silicide; the refractory metal is preferably tungsten or molybdenum; and the metal boride is preferably zirconium boride, tungsten boride, titanium boride or chromium boride.

Another aspect of the invention relates to the heterostructure for the manufacture of electronic power components, optoelectronic components or photovoltaic components comprising a support as described herein; and an active layer epitaxially grown on the seed layer made of a material characterized by having a composition of AlxInyGa_((1-x-y))N, where 0≦x≦1, 0≦y≦1 and x+y≦1. The active layer can also have a thickness of between 3 and 100 micrometers (μm).

The seed layer and the active layer are made of materials that provide a difference in lattice parameter of less than 0.005 Å. The active layer comprises a main portion in which the thickness represents between 70 and 100% of the thickness of the active layer; and in which the concentration of dopants is less than or equal to 10¹⁷ cm⁻³.

The CTE of the support substrate material is between a minimum value that is less than 0.5·10⁻⁶ K⁻¹ below the CTE of the material of the main portion of the active layer and a maximum value that is no more than 0.6·10⁻⁶ K⁻¹ above the CTE of the material of the main portion of the active layer at the temperatures used for epitaxially forming the active layer. Thus, crack-free active layers can be obtained.

The active layer has a dislocation density of less than 10⁸ cm⁻², or more preferably the active layer has a dislocation density of less than 10⁷ cm⁻².

The main portion of the active layer is inserted between a subjacent portion and a superjacent portion, and wherein the subjacent and superjacent portions each comprise a concentration of dopants greater than 10¹⁷ cm⁻³ that is of a different type than the active layer. The material of the main portion, the subjacent portion and the superjacent portion can be GaN. When the main portion of the active layer is inserted between a subjacent portion and a superjacent portion, the material of the main portion can be of a different Al_(x)In_(y)Ga_((1-x-y))N composition than the subjacent portion and the superjacent portion of the active layer. The thickness of the main portion can represent 100% of the thickness of the active layer, and the material of the main portion can be of doped n type GaN. The n type GaN of the main portion is preferably silicon doped GaN.

The main portion of the active layer and the seed layer can be comprised of the same material. In a particular embodiment, the support substrate is in molybdenum, the bonding layer is tungsten, the seed layer is GaN and the active layer is GaN.

The invention also relates to an electronic power, optoelectronic or photovoltaic component formed in or on the active layer of the heterostructure described above, and comprising at least one electrical contact on the active layer and one electrical contact on the support substrate.

The present invention also related to the method of manufacturing a support for the epitaxy of a layer of material which comprises forming a bonding layer upon a donor substrate or a support substrate; bonding the donor substrate to the support substrate such that the bonding layer is situated at the interface of the substrates; and removing a portion of the donor substrate to leave a seed layer on the support substrate for epitaxial growth of an active layer, wherein the donor substrate, the support substrate and the bonding layer are refractory materials at a temperature greater than 750° C. The material of the support substrate is preferably a metal chosen from among tungsten, molybdenum, niobium and/or tantalum and their binary, ternary or quaternary alloys, such as TaW, MoW, MoTa, MoNb, WNb or TaNb.

The support substrate is generally bonded to the donor substrate at the bonding layer(s) by molecular adhesion. The seed layer is a monocrystalline material layer adapted for the epitaxial growth of a material of composition Al_(x)In_(y)Ga_((1-x-y))N, where 0≦x≦1, 0≦y≦1 and x+y≦1.

The materials of the support substrate, the seed layer and the bonding layer are chosen such that the specific contact resistance between the seed layer and the bonding layer is less than or equal to 0.1 ohm·cm⁻². The support substrate desirably has an electrical resistivity of less than 10⁻³ ohm·cm⁻¹ and a thermal conductivity of greater than 100 W·m⁻¹·K⁻¹. The seed layer can be doped by implantation or diffusion of a dopant species.

The method can further comprise implanting ions into the donor substrate to form an embrittlement zone at a depth that is substantially equal to the thickness of the seed layer to be left on the support substrate after removing the portion of the donor substrate. The method can also be characterized in that the implantation step is carried out after the formation of the bonding layer on the donor substrate, such that the implantation is carried out through the bonding layer.

The method can further comprise measuring the surface roughness of the donor substrate and support substrate; and depositing a bonding layer on the surface of either or both substrates that have a roughness greater than 1 nm for a 5 micrometer×5 micrometer surface measured by AFM and a peak valley surface topology of greater than or equal to 10 nm, wherein the bonding layer is deposited on the surfaces having a roughness greater than 1 nm for a 5 micrometer×5 micrometer surface measured by AFM and a peak valley surface topology of greater than or equal to 10 nm; and polished until a roughness of less than 1 nm and a peak valley surface topology of less than 10 nm is reached.

The method can further comprise epitaxially growing an active layer on the seed layer to a thickness of between 3 and 100 micrometers without cracking to form a heterostructure.

The method also comprises providing the active layer with a main portion whose thickness represents between 70 and 100% of the thickness of the active layer and whose concentration of dopants is less than or equal to 10¹⁷ cm⁻³; and providing the material of the seed layer to present a lattice parameter difference with the material of the main portion of the active layer of less than 0.005 Å. The active layer is preferably a monocrystal line layer, and has a composition of Al_(x)In_(y)Ga_((1-x-y))N, where 0≦x≦1, 0≦y≦1 and x+y≦1.

The method can further comprise choosing the material of the support substrate such that the CTE of the support substrate material is between a minimum value that is less than 0.5·10⁻⁶ K⁻¹ below the CTE of the material of the main portion of the active layer and a maximum value that is no more than 0.6·10⁻⁶ K⁻¹ above the CTE of the material of the main portion of the active layer at the temperatures used for epitaxially forming the active layer.

The method can further comprise doping of the active layer, wherein the doping is carried out either during or after epitaxial growth by implantation or diffusion of dopant species.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features of this invention, its nature and various advantages will become more apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which:

FIG. 1 illustrates a heterostructure in conformance with the invention,

FIG. 2 schematically illustrates the formation of the seed layer in the donor substrate,

FIG. 3 illustrates the formation of a bonding layer on the support substrate,

FIG. 4 illustrates the assembly of the donor substrate on the support substrate before the fracture,

FIG. 5 illustrates the structure resulting from the fracture of the donor substrate,

FIG. 6 illustrates another example of a heterostructure in conformance with the invention,

FIG. 7 illustrates an example of an electronic power component formed on a heterostructure in conformance with the invention.

It is specified that, for reasons of figure readability, the relative sizes and scales of the thickness and features of the different layers have not been respected.

DETAILED DESCRIPTION OF THE INVENTION

In the present text, “layer portion” is understood to refer to a part of a layer considered in the sense of the thickness of the layer. Thus, a layer may be constituted of several stacked portions, the sum of the thicknesses of portions being equal to the total thickness of the layer. The different portions of the active layer may be in the same material, but with different doping, or rather may be in different materials of composition Al_(x)In_(y)Ga_((1-x-y))N, where 0≦x≦1, 0≦y≦1 and x+y≦1. Thus, the active layer of a PIN diode may be designed with alternating InGaN/GaN/InGaN or doped p GaN/weakly doped GaN/doped n GaN layers. The active layer for optoelectronic or photovoltaic components may be constituted of a stack of layers in different materials of composition Al_(x)In_(y)Ga_((1-x-y))N, where 0≦x≦1, 0≦y≦1 and x+y≦1.

In addition, “subjacent” designates a layer portion the farthest removed from the surface of the heterostructure and “superjacent” designates a layer portion closest to the surface of the heterostructure. By way of example, the active layer of a PIN diode comprises a single subjacent and superjacent layer on both sides of the main portion.

The present invention, according to a first aspect, relates to a support for the epitaxy of a layer of a material of composition Al_(x)In_(y)Ga_((1-x-y))N, where 0≦x≦1, 0≦y≦1 and x+y≦1, characterized in that it comprises, successively from its base to its surface; a support substrate, a bonding layer, a monocrystalline seed layer for the epitaxial growth of the layer of material Al_(x)In_(y)Ga_((1-x-y))N, and in that, the material of the support substrate presents an electrical resistivity of less than 10⁻³ ohm·cm and a thermal conductivity of greater than 100 W·m⁻¹·K⁻¹, and the seed layer is in a material of composition Al_(x)In_(y)Ga_((1-x-y))N, where 0≦x≦1, 0≦y≦1 and x+y≦1, and the specific contact resistance between the seed layer and the bonding layer is less than or equal to 0.1 ohm·cm⁻², and the materials of the support substrate, the bonding layer and the seed layer are refractory at a temperature of greater than 750° C., preferably at a temperature of greater than 1000° C.

In the present text, “refractory material” is understood to refer to a material that does not deteriorate at the epitaxy temperature of the layer of material of composition Al_(x)In_(y)Ga_((1-x-y))N (whether by melting or chemical reaction with gas components), and in which the electrical and thermal conductivity characteristics are not altered at this temperature (particularly by deterioration of the interfaces with other support layers).

It is specified that, in the whole of the present text, the coefficient of thermal expansion is considered to be the linear coefficient of thermal expansion along a plane parallel to the surface of the layers at the epitaxy temperature of the active layer with relation to the bonding temperature. The vapor phase epitaxy temperature of the active layer of type AlGaInN is conventionally less than 1100° C. Bonding is generally carried out at ambient temperature.

The coefficient of thermal expansion is conventionally measured by X-ray diffraction or by dilatometry.

Electrical conductivity (or electrical resistivity) and specific contact resistance are measured by standardized methods that are well known to the person skilled in the art and detailed, for example, in the book titled “Semiconductor Material and Device Characterization” by Dieter Schöder (John Wiley & Sons). Specific contact resistance is measured, for example, by TLM (“Transmission Line Method” or “Transfer Length Method”). This method consists of depositing metal contacts with a length l and a width w, spaced apart by a distance L_(i) over the layer of semiconductor material with a thickness h. Resistance R_(i) is measured between different contacts so as to measure several resistance values for different distances L_(i) between the contacts. These values are transferred to an orthogonal mark whose axes represent the resistances R_(i) and the distances L_(i). The slope and the zero distance point of the straight line obtained by joining the points enables the resistivity of the semiconductor material and the specific contact resistance to be respectively extracted.

Thermal conductivity is measured by standardized methods well known to the person skilled in the art and are detailed, for example, in the treatise R-2-850 “Conductivity et diffusivité thermique des solides” (Conductivity and thermal diffusivity of solids) by Alain Degiovanni, published by Techniques de l'Ingénieur.

Dislocation density may be measured by transmission electron microscopy or by cathodoluminescence.

Other characteristics of the support substrate or heterostructure listed below can also be considered alone or in combination with any other characteristic(s):

-   -   the seed layer is doped with a concentration of dopants of         between 10¹⁷ and 10²⁰ cm⁻³;     -   the dopants are preferably n type dopants;     -   the material of the support substrate is a refractory metal         chosen from among tungsten, molybdenum, niobium and/or tantalum         and their binary, ternary or quaternary alloys, such as TaW,         MoW, MoTa, MoNb, WNb or TaNb;     -   advantageously, the support substrate is in TaW comprising at         least 45% tungsten or in MoTa comprising more than 65%         molybdenum;     -   the material of the bonding layer comprises polycrystalline         silicon, silicide, preferably tungsten silicide or molybdenum         silicide, tungsten, molybdenum, zinc oxide, a metal boride such         as zirconium boride, tungsten boride, titanium boride or         chromium boride, and/or indium tin oxide;     -   the bonding layer is preferentially in a material presenting an         electrical resistivity of less than or equal to 10⁻⁴ ohm·cm;     -   the seed layer presents an electrical resistivity of between         10⁻³ and 10⁻¹ ohm·cm; and     -   the active layer presents a dislocation density of less than 10⁸         cm⁻², and preferably less than 10⁷ cm⁻².

The invention also relates to a heterostructure for the manufacture of electronic power components, optoelectronic components or photovoltaic components comprising successively from its base to its surface a support such as described above and, on the seed layer of the support, a crack-free monocrystalline layer, known as the “active layer” of a material of composition Al_(x)In_(y)Ga_((1-x-y))N, where 0≦x≦1, 0≦y≦1 and x+y≦1 present a thickness of between 3 and 100 micrometers, the seed layer presenting a difference in the lattice parameter with the material of the active layer of less than 0.005 Å and the active layer presenting a main portion in which the thickness represents between 70 and 100% of the thickness of the active layer and in which the concentration of dopants is less than or equal to 10¹⁷ cm⁻³.

In a particularly advantageous manner, the CTE of the support substrate material is between a minimum value that is less than 0.5-10⁻⁶ K⁻¹ below the CTE of the material of the main portion of the active layer and a maximum value that is no more than 0.6·10⁻⁶ K⁻¹ above the CTE of the material of the main portion of the active layer at the temperatures used for epitaxially forming the active layer. CTEs for Al_(x)In_(y)Ga_((1-x-y))N generally range from 3 to 5.6·10⁻⁶ K⁻¹ on average for temperatures ranging from room temperature to 1000° C. depending on the specific temperature and composition of the Al_(x)In_(y)Ga_((1-x-y))N:

for InN (x=0, y=1), the CTE is around 3.5·10⁻⁶ K⁻¹,

for AlN (x=1, y=0), the general range is around 3 to 4·10⁻⁶ K⁻¹,

for GaN (x=y=0), the average CTE is around 5.6·10⁻⁶ K⁻¹,

for InGaN with 5-10% of indium (x=0.05 to 0.1; y=0), the average CTE is between about 5 to 5.5·10⁻⁶ K⁻¹.

Thus, the CTE of the support material can vary above about 2.5-10⁻⁶ K⁻¹ to below about 6.2·10⁻⁶ K⁻¹.

According to a particular embodiment of the heterostructure, the main portion of the active layer is inserted between a subjacent layer and a superjacent layer, each comprising a concentration of different type dopants greater than 10¹⁷ cm⁻³. The material of the main portion and of the subjacent and superjacent portions is then preferably GaN.

According to another embodiment of the heterostructure, the main portion of the active layer is inserted between a subjacent portion and a superjacent portion, each of these portions being constituted of a material Al_(x)In_(y)Ga_((1-x-y))N of a different composition.

According to another embodiment of the heterostructure, the thickness of the main portion represents 100% of the thickness of the active layer and the material of the main portion is n type doped GaN, preferably silicon doped GaN.

Preferably, the main portion of the active layer and the seed layer are constituted of the same material.

In an example of embodiment of the heterostructure, the support substrate is in molybdenum, the bonding layer is in tungsten, the seed layer is in GaN and the active layer is in GaN.

Another aspect of the invention relates to a method of manufacturing a support for the epitaxy of a layer of a material of composition Al_(x)In_(y)Ga_((1-x-y))N, where 0≦x≦1, 0≦y≦1 and x+y≦1, characterized in that it comprises the provision of a support substrate presenting an electrical resistivity of less than 10⁻³ ohm·cm and a thermal conductivity of greater than 100 W·m⁻¹·K⁻¹, the provision of a donor substrate, the substrate comprising a monocrystalline seed layer adapted for the epitaxial growth of the layer, the formation of a bonding layer on the donor substrate and/or on the support substrate, the materials of the support substrate, the seed layer and the bonding layer are chosen to be refractory at a temperature greater than 750° C., preferably at a temperature greater than 1000° C., and chosen such that the specific contact resistance between the seed layer and the bonding layer is less than or equal to 0.1 ohm·cm⁻², bonding by molecular adhesion of the donor substrate on the support substrate, the bonding layer being situated at the interface, and thinning of the donor substrate to transfer it from the seed layer to the support substrate.

According to other characteristics of the method, considered alone or in combination:

-   -   the seed layer is formed in the donor substrate by ionic         implantation so as to create in the donor substrate an         embrittlement zone at a depth substantially equal to the         thickness of the seed layer;     -   the implantation step is carried out after the formation of the         bonding layer on the donor substrate, the implantation being         carried out through the bonding layer;     -   the method comprises doping of the seed layer, the doping being         carried out by implantation or diffusion of dopant species;     -   when the roughness of the donor substrate or support substrate         is less than 1 nm for a 5 micrometer×5 micrometer surface         measured by AFM and presents a peak valley surface topology of         less than 10 nm, the bonding layer is only deposited on the         other substrate;     -   when the roughness of the support substrate is greater than or         equal to 1 nm for a 5 micrometer×5 micrometer surface measured         by AFM and presents a peak valley surface topology greater than         or equal to 10 nm, the bonding layer is deposited and polished         until a roughness of less than 1 nm and a peak valley surface         topology of less than 10 nm is reached.

Another aspect of the invention lastly relates to a method of manufacturing a heterostructure comprising a monocrystalline layer known as the “active layer” and a material of composition Al_(x)In_(y)Ga_((1-x-y))N, where 0≦x≦1, 0≦y≦1 and x+y≦1, characterized in that the method comprises:

the formation according to the method described previously of the support for the epitaxy of the layer of Al_(x)In_(y)Ga_((1-x-y))N, and

after the formation of the support, a step of growth by epitaxy of the active layer on the seed layer, without cracking, until a thickness of between 3 and 100 micrometers is obtained.

The active layer presenting a main portion whose thickness represents between 70 and 100% of the thickness of the active layer and whose concentration of dopants is less than or equal to 10¹⁷ cm⁻³ and the material of the seed layer being chosen to present a lattice parameter difference with the material of the main portion of the active layer of less than 0.005 Å.

In a particularly advantageous manner, the material of the support substrate is chosen such that its CTE is between a minimum value that is less than 0.5·10⁻⁶ K⁻¹ below the CTE of the material of the main portion of the active layer and a maximum value that is no more than 0.6·10⁻⁶ K⁻¹ above the CTE of the material of the main portion of the active layer at the temperatures used for epiaxially forming the active layer. The method advantageously comprises doping of the active layer, the doping being carried out for the epitaxy step or, after epitaxy, by implantation or diffusion of dopant species.

Preferably, the material of the support substrate is a metal chosen from among tungsten, molybdenum, niobium and/or tantalum and their binary, ternary or quaternary alloys, such as TaW, MoW, MoTa, MoNb, WNb or TaNb.

Non-limiting examples of the different aspects and embodiments of the invention will now be described in reference to the figures.

With reference to FIG. 1, the heterostructure 1 proposed in the present invention comprises an active layer 4 of a material of composition Al_(x)In_(y)Ga_((1-x-y))N, where 0≦x≦1, 0≦y≦1 and x+y≦1.

This layer 4 is called active since it is the layer in or on which the electronic power components such as MOS, J-FET, MISFET, Schottky diodes or even thyristors; or LED components, lasers, solar cells are intended to be formed.

The thickness of the active layer 4 is between 3 and 100 micrometers, and preferably on the order of 10 micrometers.

The active layer 4 is bonded onto a support substrate 10 through a bonding layer 2 whose properties will be detailed below.

The active layer 4 is formed by epitaxy on a seed layer 3 that may be the same material as that of the active layer or a different material.

The manufacturing method of such a heterostructure 1, which will be described in detail later, mainly comprises the following steps:

the provision of the support substrate 10;

the provision of a donor substrate comprising a seed layer 3 adapted for the epitaxy of the active layer 4;

the transfer of the seed layer 3 to the support substrate 10, the bonding layer 2 mentioned above being formed at the interface; and

the growth by epitaxy of the active layer 4 on the seed layer 3.

The assembly of the support substrate 10, the bonding layer 2 and the seed layer 3, which constitutes a support for the epitaxy of the active layer 4 allowing the heterostructure illustrated in FIG. 1 to be formed, presents a total electrical resistivity of less than or equal to that of the bulk GaN comprising a dopant concentration of 10¹⁸ cm⁻³, i.e. a total electrical resistivity of less than or equal to 10⁻² ohm·cm and preferentially a thermal conductivity on the order of magnitude as that of the GaN, i.e. greater than or equal to 100 W·m⁻¹·K⁻¹ so as to propose an alternative to the utilization of a bulk GaN substrate, that is difficult to find on the market and is expensive.

Thus, heterostructure 1 such as defined above presents a sufficient vertical electrical conductivity for the intended applications such as the operation of Schottky diodes.

In addition, the materials constituting heterostructure 1 and the manufacturing methods are chosen such that the heterostructure may resist epitaxy temperatures of the active layer without deteriorating the materials or their electrical and thermal properties and thus enable operation of devices equivalent to devices formed from a bulk III/N material.

Features of the support substrate according to various embodiments of the present invention will now be described.

The support substrate 10 is in a material presenting an electrical resistivity of less than 10−3 ohm·cm and a thermal conductivity of greater than 100 W·m⁻¹·K⁻¹.

The material of the support substrate 10 is also “refractory,” i.e., it presents thermal stability at active layer formation temperatures.

It presents a coefficient of thermal expansion close to that of the material of active layer 4 at epitaxy temperature as defined herein to prevent stressing of the epitaxied layer during the heating preceding growth and during cooling of the heterostructure after epitaxy.

For example, the epitaxy temperature in vapor phase by MOCVD, HVPE of the GaN and the AlN to date is around 1000° C.-1100° C., and around 800° C. for InGaN and Al_(x)In_(y)Ga_((1-x-y))N.

In fact, if the coefficient of thermal expansion of the support substrate 10 is less than (respectively, greater than) that of the seed layer 3, the seed layer 3 will be in compression (respectively, in tension) at the epitaxy temperature.

Such being the case, this constraint in tension or in compression may be harmful to the quality of the epitaxy.

In addition, if the epitaxied layer 4 is relaxed during the epitaxy and its coefficient of thermal expansion is greater than (respectively, less than) that of the support substrate 10, it will be in tension (respectively, in compression) during cooling.

Beyond a threshold thickness, the constraints in tension as in compression are likely to relax by the formation of cracks or crystalline defects in the epitaxied layer, which reduces the efficacy of the devices formed from this layer.

The sufficiently “close” character of the coefficient of thermal expansion of the support substrate with relation to that of the active layer is determined as a function of the predominant material chosen for the active layer, i.e., the material of the main portion where appropriate.

Thus, for example, for an active layer 4 in GaN (whose coefficient of thermal expansion is on the order of 5.6·10⁻⁶ K⁻¹), the support substrate 10 may present a coefficient of thermal expansion of between 5.1·10⁻⁶ and 6.1·10⁻⁶ K⁻¹ at the epitaxy temperature of the active layer.

In general, it is considered that the coefficients of thermal expansion of the active layer (noted CTE active layer) and of the support substrate (noted CTE support) must be linked by the following relationship:

CTE active layer: 0.5·10⁻⁶ K⁻¹≦CTE support≦CTE active layer+0.6·10⁻⁶ K⁻¹

The support substrate 10 is metal-based and is preferably chosen from among tungsten (W), molybdenum (Mo), niobium (Nb) and/or tantalum (Ta) and their binary, ternary or quaternary alloys, such as TaW, MoW, MoTa, MoNb, WNb or TaNb.

In particular, the TaW alloy comprising at least 45% tungsten (preferably 75%) is likely to present a coefficient of thermal expansion in accordance with that of GaN while possessing good thermal and electrical conductivity properties.

The MoTa alloy comprising more than 65% molybdenum is also suitable.

The characteristics of some materials are found in Table 1 herein, for indicative purposes.

TABLE 1 Material Characteristics CTE at ambient CTE at Thermal Electrical temperature 1000° C. conductivity resistivity (10⁻⁶ K⁻¹) (10⁻⁶ K⁻¹) (W/m · K) (microohm · cm) Mo metal 4.8 5.6-6.0 140 5 W metal 4.5 5.1 165 5 Ta metal 6.3 7.0 54 13 Nb metal 7.3 8.3 54 15 GaN 3.5 5.6 150

The support substrate 10 is obtained, for example, by sintering (for the W for example) or by hot pressing.

Depending on the nature of the material of the support substrate 10 and the epitaxy temperature of the active layer 4 (for a material of the AlInGaN type, this varies between 800 and 1100° C.), evaporation or diffusion of the support substrate may be produced, with the effect of contaminating the active layer.

In this case, one may encapsulate the support substrate 10 by a protective layer (not represented) in view of the application of thermal budgets of active layer growth. For example, a layer of silicon nitride or aluminum nitride may be deposited on the rear face and the lateral faces of the support substrate by a CVD or PVD deposition (respectively “Chemical Vapor Deposition” and “Phase Vapor Deposition”).

Features of the seed layer according to various embodiments of the present invention will now be described.

The monocrystalline seed layer 3 is in a material Al_(x)In_(y)Ga_((1-x-y))N, where 0≦x≦1, 0≦y≦1 and x+y≦1 and is electrically conductive. The layer is obtained by transfer to the support substrate 10 from a donor substrate 30, which may be bulk or rather formed of several layers (for example, a layer of GaN deposited on a sapphire support).

The material of the seed layer is refractory, i.e., it does not deteriorate during epitaxy of the active layer.

The seed layer 3 transfer method is obtained by a step of bonding the donor substrate 30 by molecular adhesion on the support substrate 10 followed by a step of thinning the donor substrate 30 to obtain the seed layer 3. This thinning may be carried out by any technique adapted to the materials utilized and whose implementation is well known to the person skilled in the art, such as chemical mechanical polishing CMP, grinding, laser irradiation at the interface between two layers constituting the donor substrate or preferably a SMART CUT™ type method. A detailed description of this method may, for example, be found in patent EP 05 33 551 or in the book “Silicon-On-Insulator Technology Materials to VLSI”, by Jean-Pierre Colinge, 2nd edition, published by Kluwer Academic Publishers, pages 50 and 51.

Schematically, with reference to FIG. 2, the seed layer 3 is defined in the donor substrate 30 by carrying out an implantation of ionic species (for example, hydrogen) in which the implantation peak of the species has a depth substantially equal to the desired thickness for the seed layer 3. The zone with the maximum implanted species concentration constitutes an embrittlement zone 31 that may be fractured by the application, for example, of an adapted thermal budget.

The thickness of the seed layer is preferentially between 100 nm and 500 nm.

The material chosen for the seed layer 3 preferably presents a lattice parameter adapted to the epitaxial growth of the active layer 4.

preferably a material presenting a difference in lattice parameter with the material of the active layer 4 of less than 0.005 Å is chosen.

For example, one may thus carry out an epitaxy of InGaN at 1.4% In on a seed layer of GaN, or rather an epitaxy of AlGaN at 6.5% Al on a seed layer of GaN.

According to another example, for epitaxy of an active layer of GaN of a thickness of between 3 and 100 micrometers, one may remove a seed layer from a bulk substrate of GaN.

Preferably, the seed layer 3 is removed from the N polarity face of the donor substrate 30 such that the layer bonded to the support substrate 10 presents an exposed face of Ga polarity, from which the resumption of epitaxy is easier with current techniques.

Nevertheless, one may also remove the seed layer from the Ga polarity face of the donor substrate so as to carry out epitaxy on the N polarity face of the seed layer or carry out a double transfer so as to obtain an exposed face of the seed layer of Ga polarity.

Ideally, the material of the seed layer 3 is the same as that of the epitaxied active layer 4 (this is then homoepitaxy).

It then presents the same crystalline structure and, provided that the support substrate does not lead to constraints in the seed layer, there is no lattice parameter difference between the material of the seed layer and that of the active layer, which prevents the formation of new defects in the epitaxied crystal.

Homoepitaxy (active layer of GaN epitaxied on a seed layer of GaN) thus enables the lowest possible dislocation density to be obtained in the active layer.

In the case of homoepitaxy, the seed layer may be distinguished from the active layer by a difference in resistivity of these two layers when the active layer and the seed layer present different doping. In the absence of doping, it is sometimes possible to distinguish the interface between the two layers by TEM (Transmission Electron Microscopy).

In addition, the seed layer 3 advantageously presents an electrical resistivity of between 10⁻³ and 0.1 ohm·cm.

This resistivity is preferably the lowest possible resistivity, approximately 10⁻³ ohm·cm, in order to facilitate making ohmic contact with the support substrate, which corresponds to a concentration of dopants of between 10¹⁷ and 10²⁰ cm⁻³.

The material of the seed layer 3 and its level of doping are chosen so as to prevent constituting an electrical conduction barrier between the active layer and the support substrate of the final heterostructure.

The desired dopant may already be present in the layer removed from the donor or doping may be carried out by diffusion or implantation of dopant species before epitaxy of the active layer 4, depending on the desired device. Preferably, this n type doping (for example by silicon) is easier to carry out, especially as it facilitates making electrical contact with the bonding layer 2.

In addition, the seed layer 3 is sufficiently thin (between 100 nm and 500 nm, for example) so that the effect of its coefficient of thermal expansion is negligible compared to that of the support substrate 10 and that of the active layer 4.

Features of the active layer according to various embodiment of the present invention will now be described. To respond to the conductivity criterion of heterostructure 1 and to be able to support a large electric field (i.e., on the order of 10⁶ V·cm⁻¹), the epitaxied active layer 4 of Al_(x)In_(y)Ga_((1-x-y))N comprises dopants with a concentration of less than or equal to 10¹⁷ cm⁻³, in order to disperse as much as possible the electric potential drop over the entire thickness of the layer.

Doping is obtained, for example by incorporating silicon, which leads to n type doping.

Doping may be carried out during growth of the active layer 4.

Thanks to the choice of the material of the seed layer 3, the dislocation density in the active layer is less than 10⁸ cm⁻², preferably less than 10⁷ cm⁻².

In addition, a wise choice of the support substrate 10, such as stated above, enables a thick active layer 4, free from cracks, to be obtained over a thickness of between 3 and 100 micrometers, preferably between 3 and 20 micrometers, further preferably approximately 10 micrometers.

By way of information, it is specified that the term crack in the present invention is differentiated from dislocation in that the crack in a film of crystalline material corresponds to a crystalline cleavage that extends more or less deeply in the thickness of the film, i.e., a separation of the material into two parts, which creates, on both sides of the cleavage, two free surfaces in contact with air, while the film comprising a dislocation remains continuous.

According to a preferred embodiment, active layer 4 is made of GaN.

According to a particular embodiment illustrated in FIG. 6, active layer 4 is composed of a plurality of portions of layers, that is, by going from the bonding layer to the free surface of the active layer: a portion 4 a of doped n type GaN, a main portion 4 b, whose thickness is greater than or equal to 70% of the total thickness of the active layer, of weakly doped GaN (i.e. with a concentration of less than or equal to 10¹⁷ cm⁻³) and a portion 4 c of doped p type GaN, or conversely.

According to another example, the active layer 4 of a PIN diode is constituted of a main portion 4 b of GaN, a subjacent portion 4 a of InGaN and a superjacent portion 4 c of InGaN.

As is well known to the person skilled in the art, these dopings enable the desired electrical contacts to be obtained, for example in view of producing a PIN diode.

Preferably, doping of the active layer is carried out during epitaxy, with a precursor gas such as silane for n type doping to silicon, or a precursor such as Bis Cyclopentadienyl Magnesium (Cp2Mg) (C₅H₅.2.Mg) for p type doping to magnesium.

Alternately, doping may be obtained by implantation (for example silicon for n type doping, or magnesium for p type doping) or yet, for the superjacent portion, by diffusion of species in the portion (for example silicon for n type doping, magnesium for p type doping).

Features of the bonding layer according to various embodiments of the present invention will now be described.

The bonding layer is made in one or more materials chosen such that the bonding energy between the support substrate and the implanted donor substrate is greater than the fracture thermal budget.

Competition between the fracture at the embrittlement zone 31 and the separation of substrates 10, 30 at the bonding interface, which leads to a partial, poor-quality fracture of the embrittlement zone, is thus avoided.

In particular, the bonding layer is in a refractory material that does not deteriorate at the epitaxy temperature of the active layer.

In addition, the material of bonding layer 2 and its thickness are chosen so as to minimize the electric potential drop at the bonding.

Generally, the thickness of bonding layer 2 is thus less than or equal to 1 μm.

This thickness may be greater depending on the roughness of the substrate on which it is formed that it is necessary to smooth for effective bonding; In this case, the electric potential drop will be minimized by the choice of a material presenting a lower electrical resistivity.

Preferably, bonding layer 2 is electrically and thermally conductive, and the material composing the layer preferably presents an electrical resistivity of less than or equal to 10⁻⁴ ohm·cm.

However, considering that its thickness is much less than that of the support substrate, the influence of electrical and thermal conductivities of the material constituting the layer remains limited.

It is therefore possible to choose for this layer other materials than metals.

The bonding layer 2 in addition enables low electrical contact resistance between one of its faces and the active layer on the one hand, and between its other face and the support substrate on the other hand.

It is particularly desirable that the specific contact resistance between bonding layer 2 and seed layer 3 is less than or equal to 0.1 ohm·cm⁻² in order to maintain good vertical electrical conductivity despite an interface presenting a semiconductor material.

To do this, the materials from bonding layer 2 and seed layer 3 are chosen such that their output energy, i.e., minimum energy, measured in electron-volts, necessary to detach an electron from the Fermi level of a material until a point situated at infinity outside the material, is in the same order of magnitude.

A material pair satisfying this requirement is for example GaN/W, the output energy of the n type doped GaN being approximately 4.1 eV while that of the W is approximately 4.55 eV. Another pair considered may be GaN/ZrB₂ (the output energy of ZrB₂ is approximately 3.94 eV).

A layer of titanium may also promote adhesion and making contact between the material of the seed layer in III/N material and the bonding layer, since it participates in lowering the height of the conduction barrier at the interface (the output energy of Ti is approximately 4.33 eV).

A layer of titanium with a thickness of 10 nm deposited on the material of seed layer 3 or the material of bonding layer 2 before assembly is sufficient to obtain the anticipated effect.

Thermal annealing of the two materials also enables the specific contact resistance to be improved by better interconnection of the materials at the interface.

The bonding layer is deposited on donor substrate 30 before implantation and/or on support substrate 10. The act of carrying out implantation in the donor substrate after deposition of the bonding layer prevents the risk of a fracture at the implanted zone due to thermal budget produced by the deposition.

When layers of bonding material are deposited on both the donor substrate and the support substrate, bonding layer 2 is constituted of the whole of these two layers.

It is noted that when the roughness of one of the substrates (i.e., of the donor substrate or of the support substrate) is less than 1 nm for a 5 micrometer×5 micrometer surface measured by atomic force microscopy (AFM) and when it presents a peak valley surface topology of less than 10 nm measured by optical profilometry with a Wyko type apparatus, the bonding layer only has to be deposited on the other substrate. It is observed that the measured roughness is generally similar for a surface ranging from 1 micrometer×1 micrometer to 10 micrometers×10 micrometers.

Polycrystalline silicon (p-Si) is a material of choice that adheres well to the GaN, which enables planarization of the surface of the metallic support substrate and which is easy to polish.

A surface roughness before bonding of at the most a few angstroms RMS may thus be reached.

To minimize the possible diffusion of silicon to the epitaxied layer 4, a diffusion barrier (not represented) may be provided, for example between the seed layer and the bonding layer. For example, this diffusion barrier may be a film of AlN of a few nanometers of thickness.

In the case of diffusion of silicon in vapor phase during epitaxy from the lateral walls of the layer to the outside of the structure, a layer forming a diffusion barrier of the lateral zones not covered by the p-Si layer may be provided.

Alternately, one may also form the bonding layer 2 by depositing a metal (for example tungsten or molybdenum), a metal oxide such as zinc oxide, a silicide formed ex-situ (for example SiW₂ or SiMo) or a metal boride (such as Titanium Boride TiB₂, chromium boride CrB₂, zirconium boride ZrB₂, or tungsten boride WB, WB₂) on the donor substrate 30 before implantation and/or on the support substrate 10.

In a variation, it is possible to deposit a metal on the donor substrate 30 or the support substrate 10 and to deposit a silicide or a boride on the other substrate; the bonding layer 2 will then be constituted of the combination of this metal with the silicide or the boride.

Another possibility consists of making the bonding layer 2 in indium tin oxide (ITO).

As the indium tin oxide is thermally unstable, it is preferably encapsulated before carrying out epitaxy of the active layer 4.

Table 2 provides properties of some of the materials that are suitable for bonding layer 2.

The characteristic indicated in line L1 is the melting point (expressed in ° C.); the characteristic of line L2 is thermal conductivity (in W·m⁻¹·K⁻¹); that of line L3 is the electrical resistivity (in ohm·cm) and that of line L4 is the coefficient of thermal expansion (in 10⁻⁶K⁻¹).

TABLE 2 Bonding Layer Materials p-Si MoSi₂ TaSi₂ WSi₂ NbSi₂ L1 1412 1870 2499 2320 2160 L2 130 58.9 L3 1.0 · 10⁻⁴ 2.2 · 10⁻⁵ 8.5 · 10⁻⁶ 3.0 · 10⁻⁵ 5.0 · 10⁻⁵ L4 8.12 8.8-9.54 ZrB2 WBx TiB2 CrB2 L1 3060 2385 2980 1850-2100 L2 58 64 20-32 L3 9.2 · 10⁻⁶ 4.0 · 10⁻⁶ 1.6-2.8 · 10⁻⁵ 2.1 · 10⁻⁵ L4

A non-limiting example of a first preferred embodiment comprising a bonding layer in p-Si will now be described below:

First the donor substrate 30 is prepared by the following steps (see FIG. 2):

-   -   Preparation of the N polarity face of a bulk substrate 30 of         GaN: this step involves known planarization and polishing         techniques.     -   CVD or PVD deposition on the face of layer 21 of p-Si with a         thickness of 100 to 500 nm.     -   Implantation in the donor substrate 30 of ionic species (of         hydrogen, for example) through layer 21 of p-Si. The depth of         implantation determines the thickness of the seed layer 3 of         GaN. For indicative purposes, the implantation energy is between         80 and 180 keV and the dose is between 2 and 4.10¹⁷ at/cm².     -   Polishing by CMP (Chemical Mechanical Polishing) of layer 21 to         reach a roughness compatible with bonding. Typically, the         roughness before bonding must be on the order of a few angstroms         RMS.

Second, with reference to FIG. 3, the support substrate 10 is prepared, that is in a TaW alloy containing 75% tungsten.

This preparation comprises the deposition on the support substrate 10 of a layer 22 of p-Si with a thickness of a few hundred nanometers.

The thickness of the layer 22 is adapted as a function of the morphology of the surface of the support substrate 10, such that the planarization by CMP of the layer of p-Si enables a surface roughness of a few angstroms RMS to be reached.

Then a chemical treatment prior to adhesion of the two layers 21, 22 of p-Si is carried out by hydrophilic or hydrophobic bonding.

For this purpose, the surfaces are cleaned to remove contaminants and an oxidizing or deoxidizing treatment is possibly performed on the surfaces by plasma activation, drying and exposure to atmospheres containing ozone (for oxidation). The surfaces to be bonded may also be preheated up to a temperature of approximately 200° C.

With reference to FIG. 4, the surfaces of the two layers 21, 22 of p-Si are placed in contact; therefore a bonding layer 2 of p-Si with a total thickness of 100 nm to 1 micrometer is formed.

Optionally, bonding is consolidated by a thermal treatment applied from ambient temperature to approximately 200° C. with a duration of a few minutes to 2 hours. Then the fracture thermal budget is applied with a temperature ramp of between ambient temperature and approximately 600° C.

The structure illustrated in FIG. 5 is then obtained.

The damaged material on the fractured surface is removed (i.e., the surface of seed layer 3) to reach a roughness of a few angstroms to a few nanometers RMS, adapted for a resumption of epitaxy.

The heterostructure 1 illustrated in FIG. 1 is obtained by growing the active layer 4 on seed layer 3, on the desired thickness.

The residue of the donor substrate 30 may also be recycled by removing, by ion beam etching or chemical etching, the p-Si on the non-fractured face.

A non-limiting example of a second preferred embodiment having a bonding layer in WSi₂ is described below:

First the donor substrate is prepared, which includes the following steps shown in FIG. 2.

Initially the surface is prepared by planarization and polishing of the N polarity face of a bulk substrate 30 of GaN. A deposition is then carried out on the face of layer 21 of WSi₂ silicide to a thickness of 100 to 500 nm. Annealing of the layers is implemented to form ohmic contact between the GaN and adjacent material layer(s). This annealing is carried out at a temperature of between 600 and 1200° C. for a few minutes under neutral atmosphere comprising NH₃ and has the effect of reducing the contact resistance. Implantation of ionic species, such as hydrogen, through layer 21 of WSi₂ in the donor substrate 30 to a depth determining the thickness of the seed layer 3 of GaN is conducted, where the implantation energy is typically between 80 and 180 keV. The WSi₂ is polished by CMP to reach a roughness compatible with bonding (i.e., a few angstroms RMS).

A second sequence of steps comprises, preparing the support substrate 10 in TaW comprising 75% tungsten. For this purpose (see FIG. 3), a layer 22 of WSi₂ with a thickness of a few hundred nanometers is deposited on the substrate 10.

As stated above, the thickness of the layer 22 of WSi₂ is adapted as a function of the morphology of the surface of the support substrate 10 such that the planarization by CMP of the layer 22 of WSi₂ enables a surface roughness of a few angstroms RMS to be reached.

If necessary, annealing to form ohmic contact with the TaW is carried out. The conditions of this annealing are a temperature of between 600 and 1200° C., a duration of a few minutes and a neutral atmosphere.

Planarization by CMP of the surface of the WSi₂ is then carried out. Then a chemical treatment prior to adhesion of the two layers of WSi₂ is carried out by hydrophilic or hydrophobic bonding.

For this purpose, the surfaces are cleaned to remove contaminants and an oxidizing or deoxidizing treatment is possibly performed on the surfaces by plasma activation, drying and exposure to atmospheres containing ozone (for oxidation). The surfaces to be bonded may also be preheated up to a temperature of approximately 200° C.

The surfaces of the two layers 21, 22 of WSi₂ are then placed in contact; therefore a bonding layer 2 of WSi₂ with a total thickness of 100 nm to 1 micrometer is formed.

Optionally, bonding is consolidated by a thermal treatment applied from ambient temperature to approximately 200° C. with a duration of a few minutes to 2 hours.

Then the fracture thermal budget is applied with a temperature ramp of between ambient temperature and approximately 600° C.

The damaged material on the fractured surface is removed (i.e., the surface of seed layer) by dry etching (e.g., reactive ion etching (RIE)) or chemical mechanical polishing (CMP) to reach a roughness of a few angstroms to a few nanometers RMS, adapted for a resumption of epitaxy.

The heterostructure 1 illustrated in FIG. 1 is obtained by growing the active layer 4 on seed layer 3, on the desired thickness.

The residue of the donor substrate may also be recycled by removing, by dry etching for example by RIE or wet etching (i.e., chemical etching), the WSi₂ on the non-fractured face.

A non-limiting example of a third preferred embodiment is described below:

-   -   First the donor substrate 30 is prepared by the following steps         (see FIG. 2):     -   Preparation of the N polarity face of a bulk substrate 30 of         GaN: this step involves known planarization and polishing         techniques.     -   CVD or PVD deposition on the face of layer 21 of W with a         thickness of 100 to 500 nm.     -   Implantation in the donor substrate 30 of ionic species (of         hydrogen, for example) through layer 21 of W. The depth of         implantation determines the thickness of the seed layer 3 of         GaN. For indicative purposes, the implantation energy is between         80 and 180 keV and the dose is between 2 and 4.10¹⁷ at/cm2.     -   Polishing by CMP (Chemical Mechanical Polishing) of layer 21 to         reach a roughness compatible with bonding.         Typically, the roughness before bonding must be on the order of         some angstroms RMS.

Second, with reference to FIG. 3, the support substrate 10 is prepared, which is of molybdenum.

This preparation comprises the deposition on the support substrate 10 of a layer 22 of W with a thickness of a few hundred nanometers.

The thickness of the layer 22 is adapted as a function of the morphology of the surface of the support substrate 10, such that the planarization by CMP of the layer of W enables a surface roughness of a few angstroms RMS to be reached.

Then the two surfaces of layers 21 and 22 of W are placed in contact for bonding by molecular adhesion, thus a bonding layer 2 of W with a total thickness of 100 nm to 1 micrometer is formed.

Optionally, bonding is consolidated by a thermal treatment applied from ambient temperature to approximately 200° C. with a duration of a few minutes to 2 hours. Then the fracture thermal budget is applied with a temperature ramp of between ambient temperature and approximately 600° C.

The structure illustrated in FIG. 5 is then obtained.

The damaged material on the fractured surface is removed (i.e., the surface of seed layer 3) to reach a roughness of a few angstroms to a few nanometers RMS, adapted for a resumption of epitaxy.

The heterostructure 1 illustrated in FIG. 1 is obtained by growing the active layer 4 of GaN on seed layer 3, on the desired thickness.

Components Based on the Heterostructure

The heterostructure 1 described above can then be used to form electronic power components, optoelectronic components or photovoltaic components in or on the active layer 4.

In particular, the components that can be based on said heterostructure comprise electronic power components such as MOS components, bipolar transistors, J-FET, MISFET, Schottky or PIN diodes, thyristors; optoelectronic components (e.g. Laser, LED) and photovoltaic components (solar cells).

To that end, at least one contact may be formed on the active layer 4 of the heterostructure (which represents the “front side” of the component), and at least one contact may be formed on the support substrate 10 of the heterostructure (which represents the “back side” of the component).

In the case of optoelectronic components, the active layer may be relatively thin, i.e. up to 6 micrometers.

In the case of photovoltaic or optoelectronic components, the contact formed on the active layer may be transparent or semitransparent in order to allow the transmission of the appropriate wavelength.

The skilled person is able to define an appropriate material for the contact so as to meet this requirement.

A non-limiting example of an electronic power component is described below:

FIG. 7 illustrates a vertical transistor with a vertical gate (also called a “trench gate MOSFET”) made from heterostructure 1 obtained thanks to the present invention.

On the rear face of support substrate 10, a metallic layer 100 (for example, of aluminum) may be deposited to form a drain ohmic contact. However, the conductivity of the support substrate of the invention is chosen so as to not inevitably necessitate such a contact layer.

The active layer 4 of the heterostructure successively comprises a subjacent portion 4 a of doped n-GaN, a main portion of doped p+GaN with magnesium, and two superjacent regions 4 c of doped n+GaN for source contacts with layers 200 that are, for example, in an aluminum/titanium alloy.

The two regions 4 c are deposited on both sides of a trench to form the vertical gate.

The gate trench is covered with a layer 300 of a dielectric substance (such as SiO₂ or SiN) and the trench is filled with polycrystalline silicon 400.

It is to be understood that some or all of the above described features and steps can be combined in different ways, and other variations and modifications will be apparent to those of ordinary skill in the art. It is intended that all of these embodiments, examples, variations and modifications thereon are meant to be encompassed within the spirit and scope of the present invention as set forth in the following claims. In particular, the invention may be implemented with other choices of materials, according to the criteria stated above. 

1. A support for the epitaxial growth of a layer of a material, which comprises: a support substrate that includes a bonding layer; a monocrystalline seed layer on the bonding layer made of a material and having properties sufficient to facilitate epitaxial growth of an active layer of Al_(x)In_(y)Ga_((1-x-y))N, where 0≦x≦1, 0≦y≦1 and x+y≦1; wherein the seed layer and bonding layer comprise materials that provide a specific contact resistance therebetween that is less than or equal to 0.1 ohm·cm⁻², and the support substrate, the bonding layer and the seed layer each comprises a material that is refractory at a temperature of greater than 750° C.
 2. The support of claim 1, wherein the support substrate has an electrical resistivity of less than 10⁻³ ohm·cm and a thermal conductivity of greater than 100 W·m⁻¹·K⁻¹.
 3. The support of claim 1, wherein the support substrate comprises a refractory metal selected from the group consisting of tungsten, molybdenum, niobium or tantalum and their binary, ternary or quaternary alloys.
 4. The support of claim 3, wherein the metal alloys are TaW, MoW, MoTa, MoNb, WNb or TaNb.
 5. The support of claim 4 wherein the metal allow is TaW comprising at least 45% tungsten or MoTa comprising more than 65% molybdenum.
 6. The support of claim 1, wherein the bonding layer comprises a material comprising polycrystalline silicon, a silicide, a refractory metal, a metal boride, zinc oxide, or indium tin oxide.
 7. The support of claim 1, wherein the bonding layer comprises a material having an electrical resistivity of less than or equal to 10⁻⁴ ohm·cm.
 8. The support of claim 1, wherein the seed layer has an electrical resistivity of between 10⁻³ and 0.1 ohm·cm.
 9. A heterostructure for the manufacture of electronic power components, optoelectronic components or photovoltaic components comprising: the support of claim 1, wherein the material of seed layer is Al_(x)In_(y)Ga_((1-x-y))N, where 0≦x≦1, 0≦y≦1 and x+y≦1; an active layer expitaxially grown on the seed layer is crack-free and is made of a material having a composition of Al_(x)In_(y)Ga_((1-x-y))N, where 0≦x≦1, 0≦y≦1 and x+y≦1, with a thickness of between 3 and 100 micrometers and a dislocation density of less than 10⁸ cm⁻².
 10. The heterostructure of claim 9, wherein the seed layer and active layer comprise materials that provide a difference in lattice parameter of less than 0.005 Å, and the active layer comprises a main portion in which the thickness represents between 70 and 100% of the thickness of the active layer and in which the concentration of dopants is less than or equal to 10¹⁷ cm⁻³.
 11. The heterostructure of claim 9, wherein the support substrate comprises a material that provides a CTE that is between a minimum value that is less than 0.5·10⁻⁶ K⁻¹ below the CTE of the material of the main portion of the active layer and a maximum value that is no more than 0.6·10⁻⁶ K⁻¹ above the CTE of the material of the main portion of the active layer at the temperatures used for epiaxially forming the active layer.
 12. The heterostructure of claim 9, wherein the main portion of the active layer is inserted between a subjacent portion and a superjacent portion, and wherein the subjacent and superjacent portions each comprise a concentration of dopants greater than 10¹⁷ cm⁻³, wherein the main portion, the subjacent portion and the superjacent comprises GaN and the dopant of the superjacent portion is different from the dopant of the subjacent layer.
 13. The heterostructure of claim 9, wherein the main portion of the active layer is inserted between a subjacent portion and a superjacent portion, and wherein the material of the main portion is of a different Al_(x)In_(y)Ga_((1-x-y))N composition than the subjacent portion and the superjacent portion of the active layer, and the composition of the subjacent portion is different from the composition of the superjacent portion.
 14. The heterostructure of claim 9, wherein the thickness of the main portion represents 100% of the thickness of the active layer and that the material of the main portion is of doped n type GaN, or wherein the support substrate comprises molybdenum, the bonding layer is tungsten, the seed layer is GaN.
 15. An electronic power, optoelectronic or photovoltaic component formed in or on the active layer of a heterostructure according claim 1, and comprising at least one electrical contact on the active layer and at least one electrical contact on the support substrate
 16. A method of manufacturing a support for the epitaxy of a layer of material which comprises: forming a bonding layer upon one of a donor substrate or a support substrate, or on both; bonding the donor substrate to the support substrate such that the bonding layer is situated between the donor substrate and the support substrate at an interface; and removing a portion of the donor substrate to leave a seed layer on the support substrate for epitaxial growth of an active layer, wherein the donor substrate, the support substrate and the bonding layer are refractory materials having a temperature greater than 750° C. or greater than 1000° C. and chosen to provide a specific contact resistance between the seed layer and the bonding layer that is less than or equal to 0.1 ohm·cm⁻².
 17. The method of claim 16, wherein the support substrate has an electrical resistivity of less than 10⁻³ ohm·cm⁻¹ and a thermal conductivity of greater than 100 W·m⁻¹·K⁻¹.
 18. The method of claim 16, wherein the seed layer comprises a monocrystalline material layer adapted for the epitaxial growth of a material of composition Al_(x)In_(y)Ga_((1-x-y))N, where 0≦x≦1, 0≦y≦1 and x+y≦1
 19. The method of claim 16, which further comprises implanting ions into the donor substrate to form an embrittlement zone at a depth that is substantially equal to the thickness of the seed layer to be left on the support substrate after removing the portion of the donor substrate.
 20. The method according to claim 19, wherein the implanting is carried out after the formation of the bonding layer on the donor substrate, and through the bonding layer.
 21. The method of claim 16, which further comprises: measuring the surface roughness of the donor substrate and support substrate; and depositing a bonding layer on the surface of either or both substrates that have a roughness greater than 1 nm for a 5 micrometer×5 micrometer surface measured by AFM and a peak valley surface topology of greater than or equal to 10 nm.
 22. The method of claim 16, which further comprises epitaxially growing an active layer on the seed layer to a thickness of between 3 and 100 micrometers without cracking to form a heterostructure.
 23. The method of claim 22, which further comprises providing the active layer with a main portion whose thickness represents between 70 and 100% of the thickness of the active layer and whose concentration of dopants is less than or equal to 10¹⁷ cm⁻³.
 24. The method of claim 22, wherein the active layer is a monocrystalline layer, and has a composition of Al_(x)In_(y)Ga_((1-x-y))N, where 0≦x≦1, 0≦y≦1 and x+y≦1.
 25. The method of claim 22, which further comprises choosing the material of the support substrate such that the coefficient of thermal expansion (CTE) of the support substrate material is between a minimum value that is less than 0.5·10⁻⁶ K⁻¹ below the CTE of the material of the main portion of the active layer and a maximum value that is no more than 0.6·10⁻⁶ K⁻¹ above the CTE of the material of the main portion of the active layer at the temperatures used for epiaxially forming the active layer. 